Analysis starts when a small quantity of sample is introduced as either liquid or gas into the injector, which has the dual function of vaporizing the sample and mixing it with the gaseous flow at the head of the column. The column is usually a narrow-bore tube which coils around itself with a length that can vary from 1 to over 100 m, depending upon the type and the contents of the stationary phase. The column, which can serve for thousands of successive injection, is housed in a thermostatically controlled oven. At the end of the column, the mobile phase (carrier gas), passes through a detector before it exits to the atmosphere. Some gas chromatographys models of reduced size have their own electrical supply, enabling them to operate in the field (see Figure 2.19).